wordpress: running wp-cron is ineffective

It seems like wordpress-freedombox.service isn't working as intended. Here are my findings:

• Setup a Wordpress instance from source, set define('DISABLE_WP_CRON', false); and install the classic-widgets.0.2 extension from source. The current version is 0.3, so Wordpress informs me I should update to the latest version.
• Enable automatic updates on the webui and run sudo -u www-data php --file /var/www/wordpress/wp-cron.php
• When I go back to the webui, I can confirm the command worked, and I am on the latest version.

Performing the same actions (sudo -u www-data php --file /usr/share/wordpress/wp-cron.php, different path) on a FreedomBox installation doesn't update the extension. The command returns the following warning: PHP Warning: Undefined array key "HTTP_HOST" in /usr/share/wordpress/wp-config.php on line 11. This might be a Debian bug...

Moreover, it looks like wordpress-freedombox.service is incorrectly, using a relative path for ExecStart.